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The Queen’s University professor emeritus was honoured for co-discovering that elusive particles known as neutrinos can change their identity — or “oscillate” — as they travel from the sun. It proved that neutrinos must have mass, a finding that upset the Standard Model of particle physics and opened new avenues for research into the fundamental properties of the universe.

McDonald, 72, shares the prize with Takaaki Kajita, whose Japanese collaboration made the same discovery with slightly different methods.

To measure solar neutrinos, McDonald and a 130-person international team built a massive detector in an operational copper mine southwest of Sudbury. The location allowed the experiment to be highly sensitive but created enormous logistical challenges. Construction on the Sudbury Neutrino Observatory — SNO — began in 1990. The experiment collected its first data nine years later.

“I think we all knew that if we could manage to do it, it would be a very significant measurement. And that’s the way it turned out,” McDonald said Tuesday, 10 “crazy” hours after he was awakened by a telephone call from Sweden telling him he had won the prize in physics.

SNO has since expanded to become SNOLAB, with more particle experiments underway.

“What we’re really pleased about is that we are able to do experiments that can give Canadians and our international collaborators, particularly the young people, a real eureka moment.”

The SNO detector solved a long-standing mystery in physics: the case of the missing neutrinos. Theoretical models indicated there should be far more neutrinos streaming from the sun than earthly instruments were detecting. Either we didn’t understand the sun, or we didn’t understand neutrinos — highly abundant but very elusive subatomic particles that barely interact with matter, and were thought to have no mass.

To solve this problem, McDonald and his colleagues dreamt up SNO. Deep in an INCO mine (now owned by Vale), protected from cosmic radiation constantly bombarding the earth’s surface, the scientists installed a 12-metre-wide acrylic vessel filled with 1,000 tonnes of ultra-pure heavy water. The vessel was surrounded by a geodesic sphere equipped with 9,456 light sensors. The whole thing was sunk in a 34-metre-high cavity filled with regular water.

When neutrinos hit the heavy water, an event that occurred about 10 times a day, they emitted a flash of light, which researchers could analyze to measure the particles’ properties.

Neutrinos come in three identities or “flavours”: electron, tau and muon. By 2001, SNO showed that neutrinos were switching flavours in transit. To do so, they must have mass.

Once all three flavours were measured, the sun’s neutrinos were properly accounted for. The discovery spun out generations of new science — better understanding neutrinos might solve the riddle of why the universe is dominated by matter instead of being empty, for one.

“Art McDonald is an extraordinary scientist and scientific leader,” said Pekka Sinervo, a University of Toronto particle physicist who helped provide oversight for SNO in its early days and currently chairs SNOLAB’s board of directors. “He kept the collaboration moving forward, even in dark days where nothing seemed to be going right (and there were some of those).”

The discovery “took quite a bit of ingenuity and investment in major experimental science facilities,” said physics and astronomy professor Ray Jayawardhana, York University’s dean of science and author of the book Neutrino Hunters.

Building SNO cost $73 million, not including the value of the heavy water, which would have rung in at $200 million if it hadn’t been “loaned” by AECL, Canada’s federal nuclear agency.

“It is certainly not cheap. But the investment paid off . . . here we are 15 years later being recognized for that wonderful finding,” said Jayawardhana.

McDonald grew up in Nova Scotia and earned two physics degrees from Dalhousie University. He credits his parents, his high school math teacher in Sydney, N.S., and Dalhousie professors as early inspirations. He began his career at Ontario’s Chalk River nuclear research labs before moving to Princeton in 1982.

He came back to Canada in 1989 to direct SNO and take up a professorship at Queen’s, where he has been ever since. He is still involved in research at SNOLAB, which is investigating dark matter as well as neutrino science.

“We’re very fortunate in Canada to have this opportunity with SNOLAB,” McDonald said. “It’s just marvellous to be able to stimulate the next generation of students with experiments just as cutting edge as what we were doing with SNO 20 years ago.”

Prime Minister Stephen Harper congratulated the professor in a statement.

“Dr. McDonald’s award solidifies Canada’s reputation of having some of the best and brightest scientists and many of the most respected universities and research facilities in the world,” he said.

“I am certain that Dr. McDonald’s extraordinary accomplishment will further inspire our scientific community and I hope that it will encourage young Canadians who may be considering science as a career choice across the country.”

Congratulations poured in from all over, including from the Royal Societies of both Canada and the U.K., where McDonald is a fellow; the Perimeter Institute for Theoretical Physics in Waterloo, where McDonald is on the board of directors; and the small but proud Science North, an education centre in Sudbury, where McDonald has an honorary lifetime membership.